The present invention relates generally to knee prostheses and, more particularly, to a posterior stabilized knee prosthesis which provides posterior stabilization of the reconstructed knee joint by means of a mechanical cam/follower mechanism as functional compensation for lost, resected or incompetent posterior cruciate ligament structures of the natural knee.
Known total knee prostheses can essentially be classified into three basic categories. In the first category, the articular surface of the distal femur and proximal tibia are "resurfaced" with respective metal and plastic condylar-type articular bearing components. These knee prostheses provide substantial rotational and translational freedom and require minimal bone resection to accommodate the components in the available joint space. The patella-femoral joint may also be resurfaced by a third prosthetic component, as well. The femoral, tibial and patella prosthetic resurfacing components are affixed to respective adjacent bone structure by a cementing or by a biological bone ingrowth fixation means.
The femoral component is of metallic alloy construction (cobalt-chrome or 6A14V titanium alloy) and provides medial and lateral condylar bearing surfaces of multi-radius design of similar shape and geometry as the natural distal femur or femoral-side of the knee joint. The tibial component can be made entirely of plastic (UHMWPE:ultra-high molecular weight polyethylene) or comprised of a metallic base component and interlocking plastic component. The plastic tibial bearing surface is of concave multi-radius geometry to more or less match the mating femoral condyles, depending upon the design levels of primary and secondary articular constraint. Both the femoral and tibial components are independently positioned on either side of the knee joint and are not mechanically connected or linked together, as in the case of constrained or hinged type of knee prostheses, which constitutes the secondary category of total knee prostheses.
In resurfacing types of total knee prostheses according to the first category, the tibial bearing surface geometry can assume a variety of configurations, depending upon the desired extent of articular contact congruency and associated translational (medial-lateral and anterior-posterior) and rotational (axial and varus-valgus) secondary femoro-tibial motions. These various secondary motions allow the resurfaced knee to function in a natural-like biomechanical manner in conjunction with the surrounding ligamentous and muscle structures about the knee joint. The soft tissue structures maintain the femoral and tibial bearing surfaces in contact, provide the necessary levels of force constraint to achieve knee joint stability, and functionally decelerate the principal motion in flexion-extension and secondary motions, such as axial rotation, in a controlled manner. Additionally, this functional interaction between the surrounding tissue structures and the implanted knee prosthesis minimizes abrupt motion stoppage or impact loading of the prosthetic articular surfaces, thus preventing overstressing at the component fixation interface. Examples of resurfacing types of total knee prosthetic devices are disclosed in U.S. Pat. Nos. 3,774,244 to Walker; 3,728,742 to Averill et al.; and 4,207,627 to Cloutier.
On the other hand, the mechanically linked, constrained or hinged type of knee prosthesis according to the second category provides a fixed fulcrum flexion-extension capability. The "hinged knee" therefore is usually surgically indicated in selected cases where the surrounding soft tissue structures are grossly degenerated and incapable of providing functionally acceptable knee joint stability. An example of this type of total knee prosthetic device is disclosed in U.S. Pat. No. 3,996,624 to Noiles.
In clinical situations where prosthetic knee joint reconstruction is surgically indicated in the presence of compromised posterior (tibial) stability, that is, due to absent or incompetent posterior cruciate ligament structures, a posterior-stabilized total knee device can be utilized. This type of device constitutes the third category of total knee prosthetic devices. The posterior-stabilized total knee devices essentially incorporate all of the functional features of the first category, that is, the resurfacing condylar-type of knee prostheses, in addition to incorporating a mechanical cam/follower mechanism for providing posterior (tibia-to-femur) constrainment. The cam/follower mechanism is positioned within the intercondylar space of the femoral component and provides substitutional posterior constraint, as a predesigned compensation feature for lost posterior cruciate function or for compromised posterior knee stability. Thus, the possibility of anterior dislocation of the femur is reduced.
The cam portion of the cam/follower mechanism, generally includes a convex lobe shaped surface, integrally machined or cast within a box-like structure known as the "stabilizer box", located between the medial and lateral condyle runners of the femoral component and positioned between the anterior and posterior condyles. The cam surface is generally formed within the posterior wall portion of the stabilizer box and is bounded by the superior wall on the top, the medial and lateral wall portions on the sides and the anterior portion at the front. The stabilizer box structure, thus formed, occupies a significant envelope, relative to the overall dimensions of the femoral component and therefore, requires a substantial resection of viable bone to allow its accommodation within the intercondylar sector of the distal femur.
The posteriorly positioned articular convex surface of the cam is precisely ground and highly polished. The convex cam articulates with the anteriorly positioned and posteriorly oriented follower, as the knee undergoes femoro-tibial flexion and extension articulation. The mating follower surface is machined integral within the ultra-high molecular weight polyethylene (UHMWPE) tibial component. The follower member usually consists of a relatively concave articular surface located on the posterior side of an upwardly extending post-like structure, which is positioned between the concave medial and lateral tibial plateau bearing surfaces. The resultant action of the contacting cam/follower mechanism, thus described, provides posterior stabilization or constraint of the tibial component, relative to the femoral component: generally from about mid-range to full range of flexion. Within this limited range, therefore, the stabilizing mechanism essentially simulates the functional contribution of the natural posterior cruciate ligaments attached between the anterior femur and posterior tibia aspects of the knee joint. Additionally, since the cam/follower surface geometry is generally non-congruent, the mechanism can be designed to produce posterior roll-back of the femoro-tibial articular contact, simulating the natural biomechanical displacement characteristics of the natural knee.
Examples of posterior-stabilized total knee prostheses of the type described above, are disclosed in U.S. Pat. Nos. 4,209,861 to Walker; 4,298,992 to Burstein et al.; 4,213,209 to Insall et al.; and 4,888,021 to Forte et al. Each of the devices described in the above Patents incorporates a UHMWPE tibial component with a pair of medial and lateral concave plateau bearing surfaces and a metal alloy femoral component with mating multi-radius condylar runners which ride on the bearing surfaces. Both prosthesis surface geometries approximate the articular design of the natural knee. The articulation of the femoral condyles with the tibial plateau bearing surfaces allows primary femoro-tibial flexion and extension, and secondary (freedom) motions of axial and varus-valgus rotations and anterior-posterior and medial-lateral translations. The knee joint reaction forces during primary or secondary motion are principally supported by the tibial bearing surfaces, and to some extent by the cam/follower surfaces, and are transferred to the underlying fixation interfaces and adjacent supportive bone structures.
Additionally, the UHMWPE tibial component incorporates an upwardly extending post-like structure which is positioned between the plateau bearing surfaces, slightly anterior of the component mid-line. The generally concave follower surface is integrally machined on the posterior-side of the post. With the femoral and tibial knee components in a normally reduced, surgically implanted position, the upwardly extending tibial post extends into the stabilizer box structure located within the intercondylar space of the femoral component. Posterior tibial constraint is achieved when the posteriorly oriented face of the follower contacts the generally anteriorly oriented lobe surface of the cam.
Both the cam/follower articulation and the femoro-tibial articulation occur concurrently during knee flexion-extension. However, the commencement of cam/follower contact, and hence, commencement of posterior stabilization occurs on or about mid-flexion range for the devices described in U.S. Pat. Nos. 4,213,209 and 4,298,992 and near the onset of knee flexion for the device described in U.S. Pat. Nos. 4,888,021 and 4,209,861. However, as to the former Patents, it is noted that there are forces acting over the entire range of motion, which are not accounted for in these Patents.
Further, in each of the above devices, the existence of a relatively large stabilizer box at the mid-portion of the femoral component requires resection of a significant block of viable intercondylar bone to accommodate implantation of the femoral component prosthetic device. Additional surgical instrumentation is required and the surgical procedure is somewhat more complicated compared to a conventional condylar-type knee resurfacing device. Furthermore, in each instance, the stabilizer box member has medial and lateral side walls and anterior and posterior walls. These surfaces can contact the upwardly extending tibial post during inadvertent severe excursions of secondary knee motion, that is, axial and varus-valgus rotations and medial-lateral translation, and hence, can function to constrain these movements within certain limits, commensurate with design dimensional clearances. While these devices have been effective in constraining these types of motion excursions and therefore, effective in providing a high degree of controlled femoro-tibial stability, the resulting force reactions occurring between the stabilizer box surfaces and tibial post can produce periodic and severe (moment and torque) loading at the fixation sub-structure interfaces, which can cause complications related to component loosening.
Another type of posterior stabilized knee prosthesis is described in U.S. Pat. No. 4,892,547 to Brown. This design incorporates a cam/follower mechanism having a low profile and requires no resection penalty associated with the accommodation of a protruding stabilizer box structure. The heights of the tibial follower post (eminence) and femoral cam members are no higher than the thickness of the distal and posterior condyles of the femoral component. The required femoral resections are thus identical to that of a conventional resurfacing condylar-type of knee prosthesis of similar size and geometric design.
However, this Patent suffers from some disadvantages. First, although the relatively short extending tibial post is located between the plateau bearing surfaces, the cam member is integrally incorporated at a high position, between the posterior condyles of the femoral component. This knee device is therefore described as a "partially stabilized" total knee joint prosthesis, since the cam/follower mechanism only comes into contact after flexion of approximately 40.degree. has occurred, and continues until full flexion is attained. Thus, there is no posterior constrainment of the tibia relative to the femur from maximum hyperextension at about -6.degree. to approximately 40.degree. flexion. In this regard, this knee device is similar, functionally, to other devices, such as those previously described in U.S. Pat. Nos. 4,213,209 and 4,298,992. This Patent therefore does not take into account the forces acting over the entire range of motion of the knee.
Secondly, when the cam follower first contacts the cam surface at approximately 40.degree. flexion, there is initiation of mechanical posterior rollback due to the differences in articular curvature. In a normal knee, femoro-tibial rollback starts at the onset of flexion and is completed at approximately 25.degree.-30.degree. of knee flexion. This rollback provides substantially a purely rolling motion of the condyles on the tibial plateau bearing surfaces (femoro-tibial motion), and thereafter, there is a transitional motion of rolling and sliding. Therefore, it is desirable that the cam/follower contact point for start of rollback occur as early as possible in the flexion range and that completion of rollback occurs at or preferably before approximately 40.degree. flexion. The device in U.S. Pat. No. 4,892,547 furthermore describes the action of the cam/follower mechanism in producing posterior roll-back, simulating the rolling-type of posterior displacement of the femoro-tibial articular contact in the natural knee, during knee flexion. However, in U.S. Pat. No. 4,892,547, the indicated commencement of this roll-back feature occurs when the cam/follower mechanism comes into contact at approximately 40.degree. flexion and is completed after flexion to approximately 90.degree.. In the normal knee, through the complex active interaction of the anterior and posterior cruciate ligaments and other structures, the rollback displacement of the point of femoro-tibial articulation commences early in the flexion range, as aforementioned, and is essentially completed prior to 40.degree. flexion. The character of the primary articulation in flexion, after completion of the rollback stage gradually changes to a sliding and gliding mode in a manner approaching that of a fixed fulcrum posterior condyle rotation.
Third, the geometry or shape of the articular surfaces of the cam and follower members in U.S. Pat. No. 4,892,547 are not described as being congruent, and therefore, the functional contact area is small and the resultant contact stress high, when joint loading which tends to produce anterior dislocation of the femur, is imposed. Articular surface congruence of the cam and follower members is incorporated in the posterior stabilized device described in U.S. Pat. No. 4,888,021. However, this latter Patent utilizes the aforementioned box-like structure and requires a large resection area. The relative advantages of large contact areas associated with joint prosthesis of bearing congruency is frequently reported in the literature and has been adopted in a number of prosthetic devices for the knee joint, as well as, for other joints of the human body, that is, the shoulder and hip.